Sound reproducing equipment of course exists in many different types. One common example is stereo headphones which allow a user to enjoy music, radio broadcast or TV shows in a private manner without having to pay attention to other people's preferences when it comes to sound source, sound volume, etc.
To reduce disturbances from ambient noise sources, sound reproducing devices, such as stereo headphones, are sometimes provided with an “ambient noise cancellation” system, or “automatic noise cancellation” system, often abbreviated ANC. One or more microphones in the ANC system pick up the surrounding noise, and the ANC system causes one or more loudspeaker to play “anti-noise” in the ears of the user. This results in a lower noise level at the ear drums of the user.
ANC systems can be of “feed-forward” or “feed-back” types, or combinations thereof. As seen in FIG. 3a, a feed-back ANC system 300 is used in a scenario where at least one loudspeaker 302 is arranged to produce sound from an audio signal 301. The produced sound is to be perceived by the ear 11 of a user 10. The feed-back ANC system 300 uses at least one microphone 304 positioned more or less inside the ear 11 (typically, inside or close to the concha cavity of the outer ear). The microphone 304 senses the sum of the surrounding (ambient) noise and the anti-noise and produces a microphone signal 305 which includes these contents. Signal processing means 306, such as one or more signal filters, processes the microphone signal 305 and acts to provide an anti-noise output signal 307 which will counteract the ambient noise. The anti-noise output signal 307 is added at 308 to the audio signal 301 being fed to the loudspeaker 302, and the feedback loop thus formed will, over time, cancel as much as possible of the ambient noise.
It is to be noticed that the layout in FIG. 3a is schematic and does not show all possible elements of a real-life implementation of a feed-back ANC system. For instance, the audio signal components of the microphone signal 305 are typically removed prior to the signal processing means' 306 generation of the anti-noise output signal 307. Such removal may be done by functionality in the signal processing means 306 itself (having access to the audio signal 301, as seen at 301′), or by separate functionality not shown in the drawing. As is well known in the field of feed-back ANC systems, removal of the audio signal components from the microphone signal 305 may involve, for instance, the use of a least-mean-square (LMS) optimized filter together with other fixed or adaptive filters in order to account for the fixed or varying transfer function of the electro-acoustic system which includes the loudspeaker 302.
Feed-forward systems use microphone(s) outside of the ear which sense(s) only the surrounding noise. See microphone 304 in FIG. 3b. Both topologies are used commercially with various pros and cons. The anti-noise, i.e. signal 307, must be played with reversed polarity compared to the surrounding noise in order to obtain any cancellation effect. If the polarity is incorrect, the result will be more noise rather than less.
An upcoming trend is ANC in handset mode for mobile phones. The principle is the same but the anti-noise is now played by the ear speaker of the mobile phone. The effect is only perceived in one ear, and the lower frequencies are typically not possible to cancel. Nevertheless, products on the market already use this functionality. Microphones are manufactured and mounted with consistent polarity to work as intended. However, ear speakers are not always guaranteed to have consistent polarity. Furthermore, they may be mechanically designed to be rotation symmetric, which means they may fit the phone in more than one way. In order to use an ear speaker for ANC, the component manufacturer must guarantee a consistent polarity and the mobile phone assembly must be secured. This may mean that components need to have guiding pins or similar that makes incorrect mounting impossible. Off-the-shelf components may not be possible to use.